The primary objective of orthopaedic instrumentation is to maintain anatomical alignment of bone segments by sharing the loads acting on the bone, usually until bone fusion takes place. Current orthopaedic implant memory locking devices are variations of a design, in which some kind of nut is used to lock the supporting portion into the head of a bone anchor to form a coupling. For instance, in spinal instrumentation, various fastening mechanisms such as threaded fasteners or rivet fasteners have been used to secure facilitate securement of bone anchors to the connective structures. These components provide necessary stability both in tension and compression to achieve immobilization. However, it is well known that the threaded fastener can loosen under the influence of cyclic loadings that are commonly encountered by the spinal column. Therefore, the mechanical stability of the spinal implant can degrade. Also, wear between the supporting portion and the memory locking device (called fretting) is expected, which generates particulate debris that are considered to be associated with post-operation complications such as implant-induced osteolysis, pseudarthrosis, subacute low-grade implant infection, late operative site pain and abnormal metal concentration in serum. See, for example, Bullmann, et al., Spine 2003; 28(12):1306-1313; Wang, et al., Spine May 1, 1999; 24(9):899; Senaran, et al., Aug. 1, 2004; 29(15):1618-1623; and Kasai, et al., Spine 2003; 28(12):1320-1326.
U.S. Pat. Nos. 6,210,413 and 6,254,602 disclose shape memory locking devices for use in orthopedic repair. The mechanism of their locking devices depends on the interaction of compression member (20 in FIG. 1 of '602 and 24a and 24b in FIG. 2 of '413) and locking member (22 in FIG. 1 of '602 and 26a and 26b in FIG. 2 of '413). Only these two members are made of shape memory alloy and have shape memory effect and super-elasticity. The compression member and locking member generate a locking force to push the coupling member (18 in FIG. 1 of '602). Heating the locking and compression members tightens the coupling and connecting portions. However, there is no means of releasing the portions or retightening the connection materials formed of materials such as nickel-titanium (nitinol or NiTi) shape-memory-alloy, without the use of typical threaded fastening technique.
The shape-memory phenomenon is that a material can exhibit one shape at a cold temperature and another shape after being heated to a higher temperature; see Liu, et al., Materials Transactions 1996; 37(4):691-696. The material is in its original shape at the higher temperature. When being cooled to a lower temperature, the material retains its original shape but changes the structure to martensite (stable phase at the lower temperature), where the material can be easily deformed into different shapes at the lower temperature. Upon heating, the material changes back to austenite (stable phase at the higher temperature), where the deformation is recovered and the shape is restored (one-way shape memory). Alloys can also have two memories (two-way shape memory) that exhibit a reversible effect, with heat causing the change in shape which can be reversed by cooling; see Liu, et al. 1996; Liu, et al., Acta Materialia 1999; 47(1):199-209.
Super-elasticity (SE) or is a property that the material can exhibit a constant recovery force when it is under deformation at a temperature at or above the austenitic phase transition temperature. See Liu, et al., Acta Materialia 1997; 45(11):4431-4439.
It is therefore an object of the present invention to provide mechanisms for connecting orthopedic devices, specifically spinal fixation systems, which are stable and less likely to produce particulate debris.
It is a further object of the invention to provide mechanisms for connecting orthopedic devices which can be tightened, released, and reconnected in situ without the use of a coupling member
It is a still further object of the present invention to provide shape memory devices in orthopaedics that can prevent loosening and fretting at the implant interface after implantation.